Pressure sores are a major concern for wheelchair users; should the user get a sore, it can result in significant medical costs, a period of enforced bed rest, infection, and ultimately death. It is estimated that the average cost today of treating a pressure sore is on the order of $15,000. Also, various studies have attributed about 5% of the deaths of paraplegics and quadriplegic to complications from pressure sores.
Careful distribution and periodic relief of the pressure on a seated person at the seat interface is important for prevention of pressure sores. In an able-bodied person, moving in and out or around one's chair is a principal mechanism for accomplishing this. For para/quadriplegic and other persons at risk for pressure sores such as the elderly or persons with cerebral palsy multiple sclerosis, or muscular degenerative diseases generally, this may not be an option. Strategies commonly adopted for prevention of pressure sores in susceptible individuals may be categorized generally as passive and active.
Passive seat cushions distribute the load more optimally across the buttocks of the seated individual. These cushions are employed to transfer load from the higher risk areas of the buttocks, namely tissue overlying bony prominence such as the ischials, the coccyx, the sacrum, and the trochanters, onto lower risk areas such as the posterior thighs. Passive seat cushions include air flotation types, gel types, combination air/foam cushions, honeycombs, as well as simple blocks of foam. The term "foam," as used in this specification and in the appended claims, refers to a class of materials consisting of included voids (or "cells") filled with air or another fluid within the matrix of a solid, which materials exhibit some degree of resilience in that they recover some or all of their initial volume following compression and release.
For individuals at high risk of developing pressure sores, or individuals who are unable to redistribute their weight periodically on a seat, a passive seat cushion is generally not adequate to eliminate the risk. In this case, it is generally recommended to supplement the passive cushion with a tilt and/or recline seating system, or, otherwise, an attendant may assist with manual pressure relief. A tilt and/or recline method, whether powered or manual, typically lays the user onto his or her back while attendant assisted pressure relief typically involves lifting the user and repositioning the user in the chair.
A further possibility for individuals at high risk is an active seat cushion that employs pneumatic or mechanical means to cyclically relieve pressure under some portion of the anatomy. Such devices generally use positive air pressure to inflate some portion of the cushion, and, by increasing pressure in the inflated area, raise the user and decrease the interface pressure in those portions of the seat which are not inflated. The term "interface pressure," as used in this description and in the appended claims, refers to the force per unit area exerted by the weight of the seated person on the cushion material, and the equal and opposing force per unit area exerted by the cushion on the seated person's body.
1. Foam Characteristics and Pads for Seating
Foams, as defined above, are discussed in further detail in L. Gibson, Cellular Solids: Structures and Properties (Pergamon Press, 1988), which is herein incorporated by reference. The cells of a foam are, furthermore, distinguished as "open" or "closed" with respect to the flow of air into, or out of, the respective cells.
Foams are conventionally characterized in terms of "stiffness," a measure which indicates, on the basis of a standard measurement technique, how much force per unit area is required to cause a specified indentation of the middle of a test block of material in bulk. For example, the Indentation Load Deflection (ILD), specified by ASTM Standard Test D-3574-81, specifies the load causing a 25% indentation of the material.
The use of a single stiffness measure is deficient in two important respects that are relevant to the present invention. On the one hand, the single stiffness measure fails to distinguish among characteristics of the functional dependence of pressure on the displacement of the foam, in which various regimes may be discerned, as discussed in detail in the description below. Moreover, the single stiffness measure is limited to characterization of the material in bulk and does not account for the structural configuration in which the material is used. Thus, while the behavior of a bulk configuration of material may be dominated by its compression, tall columns of material, much taller than the characteristic transverse material dimension, will be dominated by buckling, or columnar collapse, of the entire column. In a third regime, so-called "short columns" exhibit material properties intermediate between those of the bulk and columnar limits. The relevant structural parameter is thus the aspect ratio of the column, namely, the ratio of the height of material normal to the surface of contact with the seated person to the narrowest transverse dimension of the column, such as its width if the cross section of the column is square.
In particular, in open-cell foams such as poly-ether and poly-ester based poly-urethanes, the pressure-displacement characteristic is non-linear. This is in contradistinction to the behavior of an ordinary spring in which the compressive or tensile displacement is proportional to the force applied, in accordance with Hooke's law. The non-linear response of open-cell foams is caused by the cellular nature of the material, and is present to some degree in all visco-elastic open-cell materials, of which foam is the most common. In addition to non-linearity, foams typically deviate from Hooke's law spring response in exhibiting hysteresis: the displacement-pressure curve varies in accordance with whether the foam is undergoing compression or recovery from compression.
Several manufacturers make seats out of more than one foam stiffness with the intention of optimizing the interface pressure with respect to the health, safety, or comfort of the seated person. Examples include U.S. Pat. No. 5,000,515 to Deview and U.S. Pat. No. 5,442,843 to Siekman et al. In addition to the use of foam in manufactured seats, foam seating materials are sold to clinicians for custom construction of seats.
The lifetime of foam cushions is severely shortened if the foam comes in contact with liquids, such as sweat or urine. Seating cushions, therefore, may employ a plastic cover to keep them dry, but the cover is apt to negatively impact the capacity of the cushion to distribute pressure and heat at the interface with the subject. Foam cushions may also be coated with silicone caulking or other waterproofing compound, but such coatings may negatively impact cushion performance.
Cushion shape may be used to improve pressure management performance, and may be customized for the user, however any benefits of specific shaping are compromised if the user is not seated as intended with respect to the cushion.
The deficiencies of foam cushions generally also characterize gel cushions, which, additionally, tend to add significantly to the weight budget of a seating apparatus and the pressure management performance of gel cushions is negatively affected by changes in ambient temperature.
2. Pneumatic Devices for Seating:
Various products use a positive pressure of air or other fluid to manipulate the properties of devices for supporting the human body. These include camping pads, air casts, the ROHO wheelchair cushion (U.S. Pat. No. 4,698,864 to Graebe), and adjustable automobile seats. Products of this sort use fluid bladders to exert pressure on particular portions of the anatomy. The device may consist solely of bladders, a series combination of foam and bladders, or a parallel combination of cushion and pneumatics. In the case of the series combination of foam and bladders, as employed in some auto seats and air casts, the bladders are used to adjust the shape and, therefore, the user's point of contact with the underlying foam cushion. In parallel applications, used in certain wheelchair cushions and camping pads, the foam is typically bonded to the bladders and provides a minimum cushioning in case air pressure is lost, and also significantly modifies the pneumatic impedance of the air bladders should the state of load on them change.
Many of the foregoing devices use a constant mass of air or other fluid, with the volume of the device and the pressure within it varying with the position and actions of the user. For example, if the pad is thick enough that the user does not bottom out, and the user sits on the inflated pad, the user displaces some of the volume of the cushion, and the internal pressure rises, perhaps considerably. As the user's weight shifts around the cushion, the internal pressure generates a balloon effect causing a feeling of instability.
Pneumatic cushions have a narrow range of parameters under which acceptable pressure management performance may be achieved due to sensitivity to ambient temperature and pressure changes over time. Additionally, if a pneumatic cushion develops a hole, it rapidly deflates, leaving the user seated uncushioned on the cushion substrate.